Wafer alignment in a conventional semiconductor manufacturing apparatus will be described with reference to FIGS. 1, 4, 6A to 6C, and 7.
When a wafer W is supplied to the semiconductor manufacturing apparatus, a mechanical alignment apparatus MA performs mechanical alignment using the circumference of the wafer W and a mark called an orientation flat or a notch (N in FIG. 4) to determine the rough position of the wafer W. The precision of the mechanical alignment is about 20 μm. The wafer W is mounted on a chuck CH by a wafer supply apparatus (not shown) to perform global alignment. In global alignment, measurement marks FXY1 to FXY4 shown in FIG. 4 are measured, thereby obtaining shifts in the X and Y directions, a rotational component, and a magnification component of the shot array. The precision of the global alignment is less than 50 nm in a current machine for manufacturing a 265-Mbit memory.
To observe a mark in global alignment, the wafer W chucked by the wafer chuck CH on a stage STG is observed with a scope SC. The scope SC comprises a microscope having two types of magnifications and a sensor. Illumination light from an illumination light source Li passes through a half mirror M1 to come incident on an alignment mark on the wafer W. Light reflected by the alignment mark passes through the half mirror M1 and is split into two light components by a half mirror M2. One light component passes through a low magnification optical system to form an image on a sensor S1. The other light component passes through a high magnification optical system to form an image on a sensor S2. The wafer stage STG is moved by a motor MS in accordance with instructions from a controller MC, while a laser interferometer LP measures the accurate position of the stage.
Images formed on the sensors S1 and S2 are photoelectrically converted, and a mark position calculation processor P calculates the position of the mark. The calculation of the mark position is performed for each of the low- and high-magnification systems. An electrical signal is photoelectrically converted by the low magnification system sensor and is converted by an A/D converter AD1 from an analog signal to a digital signal, which is stored in an image memory MEM1. An image processing unit COM1 searches the memory by pattern matching, or the like, to obtain the mark position. As the low-magnification system sensor, an area sensor, such as a CCD camera, is used. An electrical signal from the high-magnification system sensor is converted by an A/D converter AD2 from an analog signal to a digital signal, which is stored in an image memory MEM2. An image processing unit COM2 calculates the mark position for the high-magnification system. The actual mark position is determined on the basis of the image on the high magnification system sensor S2. The position of the wafer W is determined from the mark position calculated in the image processing unit COM1 and the stage position specified by the controller MC.
The reason for the measurement by the two types of sensors will be described next. FIG. 6A shows a high-magnification system field HF. The visual field for the low-magnification system is a range MF shown in FIG. 6B. It is confirmed by the low-magnification system whether a result of mechanical alignment falls within the visual field of the sensor S2. If the result falls within the visual field, an alignment result obtained by the high-magnification system is adopted. If the result falls outside the visual field, small movement amounts dx and dy for aligning within the visual field of the sensor S2 are calculated. With this operation, global alignment can be completed with high precision at high speed, in spite of any error in mechanical alignment.
After the end of the alignment, the circuit pattern of a reticle MASK on a reticle stage RSTG is projected onto the resist on the wafer W via a projection lens LENS. In exposure, a masking blade MB is set in accordance with an exposure region on the reticle MASK by a reticle reference plate PL. Light emitted by an exposure illumination device IL exposes the wafer W via the masking blade MB, reticle MASK, and projection lens LENS.
The processing flow will specifically be described with reference to FIG. 7. In global alignment, the scope SC moves to a mark FXY1 (S101) to measure the positions of marks FX1 and FY1 (S102). Measurement is performed in accordance with the flow of step S10. This flow shows a case wherein the high-magnification system sensor S2 comprises an X-measurement high-magnification system and a Y-measurement high-magnification system. In this case, a line sensor, or the like, is used. X and Y marks may be image-sensed by an area sensor. X-direction image sensing at a high magnification (S110), Y-direction image sensing at high magnification, and image sensing at a low magnification are simultaneously performed. These types of image sensings need not be performed simultaneously, but simultaneous image sensing increases the speed. Mark position calculation of FX (S113) and FY (S114) and mark position calculation at low magnification are performed, and moving amounts dx and dy for an image to be made to fall within the visual field for high-magnification detection are calculated from a result of step S115 (S116). It is determined whether the amounts fall within an allowable range (S117). If the amounts fall within the range, results calculated in steps S113 and S114 are adopted. If the amounts fall outside the range, the wafer is image sensing at a high magnification (S119), Y-direction image sensing at a low magnification (S120), and mark position calculation of FX (S121) and FY (S122) are performed.
When the position of the mark FXY1 is calculated, the scope moves to the mark FXY2 (S103). The positions of marks FX2 and FY2 are calculated in the same manner (S104). When the two accurate mark positions are obtained, the rough position of the wafer W on the chuck CH is obtained. The target positions of marks FXY3 and FXY4 are calculated again by reflecting the result (S105). When the rough position is obtained, the mark position falls within the visual field for the high-magnification system. In steps S106 and S107, the mark positions of marks FX3, FY3, FX4, and FY4 are calculated, and global alignment ends.
If an error generated upon mechanical alignment is about 20 μm, as described above, the position of the wafer W can be calculated at high speed and high precision by global alignment by the low- and high-magnification systems. However, mechanical alignment is performed, not on the stage STG, but outside the stage. For this reason, if an error generated upon mounting the wafer W by a hand, or the like, on the chuck CH is 20 μm or less, the error may exceed 20 μm in absolute value if the wafer is manufactured by another apparatus. This amount is referred to as an offset. Even if the reproduction precision of mechanical alignment is high, variations in offset may occur due to a mechanical error in each mechanical alignment apparatus. Thus, if exposure as the preprocess is performed in another apparatus, and a wafer is observed by the low-magnification system, the wafer may be placed on the chuck while a mark is so shifted as to fall outside the visual field. In this case, an alignment mark formed in another process may fall within the visual field. If the alignment mark in the above process has the same shape, it is detected by mistake, and global alignment becomes impossible.
Semiconductor manufacturing methods are recently making progress, and, in particular, planarization, which performs a polishing process called CMP is contributing to an increase in the degree of intergration, and, for this reason, a layer on the alignment mark is also polished, and a mark signal may degrade or its stability may decrease. Alignment marks tend to be optimized in accordance with these processes. The structure of each mark such as the line width, spacing, and three-dimensional pattern has been changed to leave the optimum alignment mark. In general, marks are determined in the prototyping stage. In flexible manufacturing, however, semiconductors are often manufactured in quantity, without optimization.
Recent alignment tends to form a plurality of sets of alignment marks in one region (exposure region) from a set of X and Y alignment marks. This aims at performing deformation correction of the exposure region and increasing the precision by an averaging effect by measurement at a plurality of marks. In this case, the span of the exposure region should be increased as much as possible to increase the precision. More specifically, alignment marks tend to be formed at the four corners of the exposure region.
As described above, recent alignment mark formation has the following tendencies.
(1) The number of alignment marks increase with an increasing number of processes.
(2) The number of alignment marks increases for the optimization of the marks with increasing susceptibility of their structures.
(3) The number of marks increases for an increase in measurement precision.
For these reasons, the number of alignment marks in a scribe line in which alignment marks can be formed increases, and two or more alignment marks are often observed in the visual field for the alignment scope, as shown in FIG. 6C.
To solve the above-mentioned problems, the following method has conventionally been considered.
More specifically, alignment marks are separated from each other, such that two or more of them fall within the same visual field.
As describe above, since the number of alignment marks tends to increase, only optimized marks are formed in this method, if possible. In some cases, optimization needs to be performed for each lot, which is impractical. In a semiconductor manufacturing line, such troublesome operation is avoided. Even if this lot-by-lot optimization is possible, there is a limit on the number of alignment marks to be formed.
Under the circumstances, to solve this problem, Japanese Patent Application Laid-Open No. 2001-274058, discloses a method of identifying a target mark by, for example, forming identification marks around each alignment mark of deforming part of each alignment mark. However, in this method, if the target mark is greatly damaged after having undergone a wafer process step, such as CMP, the target mark cannot be identified.
In addition, measurement resolution may be insufficient when a detection apparatus having a wide visual field for giving a higher priority to the mark identification is to perform high-precision measurement. If the identification is possible, high-quality signals required for high-precision measurement are not always obtained.